Fiber material and method for manufacturing the same

ABSTRACT

A fiber material contains an assembly of a plurality of nanofiber coated polymer fibers in which a polymer fiber serving as a core fiber is coated with polymer nanofibers, in which the polymer fibers and the polymer nanofibers are fusion-bonded and two or more of the polymer fibers are fusion-bonded to each other in at least one portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/604,489, filed Jan. 23, 2015, which claims the benefit of JapanesePatent Application No. 2014-012756, filed Jan. 27, 2014, each of whichis hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a fiber material and a method formanufacturing the same and more specifically relates to a fiber materialcoated with nanofiber and a method for manufacturing the same.

Description of the Related Art

In recent years, a fiber material has drawn attention as a materialhaving a large specific surface area. In particular, an increase inspecific surface area based on surface treatment of fibers and adevelopment of a material employing the same have been energeticallyexamined in and outside Japan.

Japanese Patent Laid-Open No. 2009-219952 discloses a fiber material forfilters having a large specific surface area in which nanofibers towhich a functional group is given are ejected to a core fiber whileapplying a high voltage to coat the core fiber. Herein, an increase inspecific surface area is achieved by performing surface treatment offibers by coating with nanofibers. More specifically, the fiber materialcoated with nanofibers (nanofiber coated fiber material) which ismanufactured by spouting the nanofibers to the core fiber while windinga thin fiber serving as the core fiber has a large specific surfacearea. Therefore, it is described that a bobbin type filter havingexcellent filtration performance is obtained by forming fiber bundles bytwisting and bundling a plurality of nanofiber coated fiber materials,and then winding the fiber bundles around the periphery of a cylindricalbody having a cylinder wall with transmission properties.

However, the nanofiber coated fiber material disclosed in JapanesePatent Laid-Open No. 2009-219952 has a constitution that the core fiberand the nanofibers coating the same or the core fibers are physicallytwisted. Therefore, the nanofiber coated fiber material has had problemsin the following application. First, the peeling resistance between thenanofibers and the core fiber is low, and thus the nanofibers are easilypeeled and detached due to an external factor, such as rubbing, so thatthe specific surface area of the fiber material decreases. Second, themechanical strength of the fiber material is low, and thus the fibersare easily loosened, which is disadvantageous for long-term use.

SUMMARY OF THE INVENTION

The present invention provides a fiber material containing an assemblyof a plurality of nanofiber coated polymer fibers in which a polymerfiber serving as a core fiber is coated with polymer nanofibers, inwhich the polymer fiber and the polymer nanofibers are fusion-bonded andtwo or more of the polymer fibers are fusion-bonded to each other in atleast one portion.

The present invention also provides a method for manufacturing a fibermaterial including a spinning process including coating a polymer fiberas a core fiber containing a polymer material with polymer nanofiberscontaining a polymer material to obtain a nanofiber coated polymerfiber, and a fusion-bonding process including fusion-bonding the polymerfiber and the polymer nanofibers and also fusion-bonding two or more ofpolymer fibers in at least one portion.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic views illustrating one embodiment of afiber material of the present invention.

FIG. 2 is a schematic view illustrating one embodiment of a method formanufacturing a fiber material of the present invention.

FIG. 3 is an optical microscope photograph of a fiber material ofExample 5 of the present invention.

DESCRIPTION OF THE EMBODIMENTS

The present invention has been made in view of such a background art andprovides a fiber material coated with nanofibers which is excellent inpeeling resistance between nanofibers and a core fiber and has highmechanical strength and a method for manufacturing the same.

Hereinafter, embodiments of the present invention are described.

Fiber Material According to the Present Invention

First, a fiber material according to the present invention is described.

The present invention provides a fiber material containing an assemblyof a plurality of nanofiber coated polymer fibers in which a polymerfiber serving as a core fiber is coated with polymer nanofibers, inwhich the polymer fibers and the polymer nanofibers are fusion-bondedand two or more of the polymer fibers are fusion-bonded to each other inat least one portion.

For example, the fiber material according to the present inventioncontaining an assembly of a plurality of nanofiber coated polymer fibersformed by coating a polymer nanofiber as a core fiber with polymernanofibers, in which the polymer fiber and the polymer nanofiberscoating the polymer fibers are fusion-bonded and two or more of thepolymer fibers forming the fiber material are fusion-bonded to eachother in at least one portion.

With respect to the fiber diameter of the core fiber and the coatingfibers coating the core fiber in the present invention, the averagediameter of the fibers is 1 μm or more and 50 μm or less and 1 nm ormore and less than 1 μm, respectively. For the core fiber, known fibermaterials, e.g., carbon fiber materials and fiber materials, can be usedsingly or in combination of two or more kinds thereof as appropriate.

In this embodiment, among the fibers, fibers containing polymermolecules and having an average diameter of 1 μm or more and 50 μm orless are referred to as polymer fibers, fibers having an averagediameter of 1 nm or more and less than 1 μm are referred to asnanofibers, and, in particular, fibers containing polymer molecules andhaving an average diameter 1 nm or more and less than 1 μm are referredto as polymer nanofibers.

The fibers according to the embodiment of the present invention have alength longer than the thickness of the fibers. The cross-sectionalshape of the fibers is not particularly limited and may be a circle, anoval, a quadrangle, a polygon, a semicircle, and the like or may not bean exact shape and may have different shapes in an arbitrary crosssection.

The thickness (diameter) of the fibers refers to the diameter of thecircle of the cross section in a fiber in which the cross-section of thefiber is a cylindrical shape and, in other cases, refers to the lengthof the longest straight line passing through the center of gravity inthe fiber cross section. The length of the polymer nanofiber is 10 timesor more the thickness thereof.

A structure containing fibers has a feature that the specific surfacearea increases with a reduction in diameter. Therefore, the structure iseffective because when the fiber diameter is smaller, a fiber materialhaving a large specific surface area is obtained. In addition thereto,nanofibers having a nanosized diameter demonstrate an effect referred toas supramolecular arrangement effect in which, in a process ofmanufacturing the nanofibers, additives added to the nanofibers areuniformly arranged in the longitudinal direction in an ultrafine spaceand also, in the case where the nanofibers are polymer molecules(polymer nanofibers), the molecular chains of the polymer molecules areuniformly arranged. As a result, fibers having high mechanical strengthare obtained.

Next, the fiber material coated with the polymer nanofibers of thepresent invention is described with reference to FIGS. 1A and 1B.

FIGS. 1A and 1B are schematic views illustrating one embodiment of thefiber material of the present invention. FIG. 1A illustrates a schematiccross sectional view of one nanofiber coated polymer fiber and FIG. 1Billustrates a perspective view of a fiber material. The figures includea nanofiber coated polymer fiber 1, a polymer fiber 2, a polymernanofiber 3, and a fiber material 4.

The fiber material 4 coated with the nanofibers of this embodiment hasinevitably unevenness on the surface and contains the nanofiber coatedpolymer fibers 1, and therefore has a large specific surface area. Oneof the nanofiber coated polymer fibers 1 in the fiber material of thepresent invention has a constitution in which the polymer nanofibers 3at least containing a polymer material are fusion-bonded to the surfaceof the core fiber 2 at least containing a polymer material. As a result,the peeling resistance between the polymer nanofibers 3 and the polymerfiber 2 as the core fiber is high, so that the polymer nanofibers 3 arenot easily peeled and detached due to an external factor, such asrubbing, to reduce the specific surface area of the fiber material.

The specific surface area and the surface unevenness in the nanofibercoated polymer fibers depend on the coating ratio of the polymernanofibers, the number of the polymer nanofibers coating the polymerfibers, and the like, which may be selected as appropriate according toa desired property.

In the fiber material of the present invention, the number, the intervalof adjacent fibers, and the number of laminations of the nanofibercoated polymer fibers 1 in an arbitrary cross section can be suitablyselected according to a desired property of the fiber material. Forexample, FIG. 1B illustrates a constitution in which a plurality ofnanofiber coated polymer fibers 1 are disposed in a random shape and thenanofiber coated polymer fibers 1 are fusion-bonded to each other in atleast one portion.

More specifically, a plurality of nanofiber coated polymer fibersadjacent to each other which form the fiber material 4 form a firm andflexible network by fusion-bonding to each other in at least oneportion. As a result, the fiber material obtained by the presentinvention has high mechanical strength, is free from ease loosening ofthe core fibers, and is advantageous for long-term use.

From the description above, the present invention can provide the fibermaterial in which the polymer nanofibers coat the polymer fiber in whichthe peeling resistance between the polymer fiber as the core fiber andthe polymer nanofibers is high, the mechanical strength of the fibermaterial is high, and the specific surface area is large.

The fiber material of the present invention can be a fiber materialhaving a large specific surface area which can be used over a longperiod of time even when an external factor, rubbing, is applied, andtherefore can be suitably utilized, for example, as a friction chargingmaterial in a static electricity generator and a particle electric fieldseparator.

Polymer Fiber as Core Fiber

The polymer fiber as the core fiber in the present invention is notparticularly limited and may be a fiber at least containing a polymermaterial, and an organic polymer is suitable. As the organic polymer,known organic polymer materials can be used singly or in combination asappropriate. Moreover, the organic polymer may be a polymer materialcontaining fine particles or a known filler and the like and can beconstituted by combining the same as appropriate.

The polymer fiber in the present invention contains at least one or morekinds of polymers and has a length longer than the thickness of thepolymer fiber.

The thickness of the polymer fiber is suitably larger than that thepolymer nanofiber coating the polymer fiber and the average diameter issuitably 1 μm or more and 50 μm or less. The average diameter is moresuitably 10 μm or more and 50 μm or less.

In order to coat the polymer fiber with the polymer nanofibers, when thecore fiber is thinner than the polymer nanofibers, the handling isdifficult from the viewpoint of manufacturing and also it is sometimesdifficult to excellently coat the periphery of the core fiber with thepolymer nanofibers. Therefore, the thickness of the polymer fiber issuitably larger than that of the polymer nanofiber for coating thepolymer fiber and more suitably 10% or more larger than that of thepolymer nanofibers and the average diameter is suitably 1 μm or more.

In addition thereto, in order to obtain a fiber material having a largespecific surface area, the average diameter of the polymer fiber issuitably 50 μm or less because a large specific surface area tends to beeasily obtained by coating the core fiber with the polymer nanofiberswith a small diameter.

The cross-sectional shape of the polymer fiber is not particularlylimited and may be a circle, an oval, a quadrangle, a polygon, asemicircle, and the like or may not be an exact shape and may havedifferent shapes in an arbitrary cross section.

The thickness of the polymer fiber refers to the diameter of the circleof the cross section in one in which the cross section of the polymernanofiber has a circular shape but, in other cases, the thickness refersto the length of the longest straight line passing through the center ofgravity in the fiber cross section. The length of the polymer nanofibersis 10 or more times the thickness thereof.

The polymer material of the polymer fibers in the present invention isnot particularly limited insofar as the material can at least partiallymelt and forms a fiber structure, and organic materials, such as resinmaterials, inorganic materials, such as silica, titania, and claymineral, or materials obtained by hybridizing the organic materials andthe inorganic materials may be used.

Examples of the polymer material include, for example, polyolefinpolymers, such as fluorine containing polymers, e.g.,tetrafluoroethylene and polyvinylidene fluoride, for example,polyvinylidene fluoride (PVDF), a copolymer (PVDF-HFP) of PVDF andhexafluoro propylene, polyethylene, and polypropylene; polystyrene (PS);polyarylenes (aromatic polymers), such as polyparaphenylene oxide,poly(2,6-dimethyl phenylene oxide), and polyparaphenylene sulfide; thosein which a sulfonic acid group (—SO₃H), a carboxyl group (—COOH), aphosphoric group, a sulfonium group, an ammonium group, or a pyridiniumis introduced into polyolefin polymers, polystyrenes, polyimides, andpolyarylenes (aromatic polymers); perfluoro sulfonic acid polymers,perfluoro carboxylic acid polymers, and perfluoro phosphoric acidpolymers in which a sulfonic acid group, a carboxyl group, a phosphoricgroup, a sulfonium group, an ammonium group, or a pyridinium group isintroduced into the skeleton of polymers of polytetrafluoroethylene andfluorine containing polymers, polybutadiene compounds; polyurethanecompounds, such as elastomer or gel; silicone compounds; polyvinylchlorides; polyethylene terephthalate; nylon; and polyesters (PES), suchas polyarylate, polycapro lactone (PCL) and polylactic acid which arebiodegradable polymers, ethers, such as polyethylene oxide (PEO) andpolybutylene oxide, and polyethylene terephthalate (PET). Thesesubstances can be used singly or in combination of two or more kindsthereof, may be functionalized, or may be copolymerized with otherpolymers. In the case of polymer materials, such as polyimide,polyamide, polyamide imide (PAI), and polybenzimidazole (PBI) which arehard to be melted, the polymer materials can be used in combination withthermoplastic resin, for example.

Examples of the inorganic materials include oxides of Si, Mg, Al, Ti,Zr, V, Cr, Mn, Fe, Co, Ni, Cu, Sn, and Zn and, more specifically, thefollowing metal oxides are mentioned. Silica (SiO₂), titanium oxide,aluminum oxide, alumina sol, zirconium dioxide, iron oxide, chromiumoxide, and the like can be mentioned. Clay mineral, such asmontmorillonite (MN), can also be used.

When the inorganic materials are contained in the polymer fibers, themechanical strength tends to remarkably improve by fusion-bonding thefibers. Therefore, the blending of the inorganic materials is suitablefrom the viewpoint of an improvement of durability.

In the present invention, when the polymer fibers at least contain thepolymer material constituting the polymer nanofibers, the compatibilitythereof is high, and thus the fibers tend to be firmly fusion-bonded.

The description “the polymer fibers at least contain the polymermaterial constituting the polymer nanofibers” in the present inventionmeans that polymer material formed with the same skeleton structure asthe skeleton constituting the polymer nanofibers may be at leastcontained in the polymer fibers and it is not necessary to be the samepolymer material.

Due to the material configuration described above, there is a tendencyfor the mechanical strength of the fiber material in the presentinvention to remarkably improve. Therefore, the material configurationis suitable from the viewpoint of an improvement of durability.

When the melting temperature (Melting point: Tm₁) forming the polymerfiber is less than Tm₂ of the polymer material forming the polymernanofibers and the difference is large, the polymer fibers and thepolymer nanofibers coating the same can be fusion-bonded bypreferentially melting the polymer fiber side. More specifically, theshape (unevenness) of the polymer nanofibers is maintained and an effectof increasing the specific surface area in the fiber material based onthe coating with the polymer nanofibers is easily demonstrated.Therefore, the fusion-bonding manner is suitable.

Herein, it is suitable that the temperature difference (Tm₂−Tm₁) in themelting points of the polymer fibers and the polymer nanofibers is 5° C.or higher and suitably 30° C. or higher from the viewpoint of thermalcontrol. More specifically, when the temperature difference is less than5° C., the shape of the polymer nanofibers tends to be difficult tomaintain.

It is more suitable that the melting point (Tm) of the polymer materialforming the polymer fibers is equal to or less than the glass transitionpoint (Tg) of the polymer material forming the polymer nanofibersbecause the shape of the polymer nanofibers is very easily maintained inthe fusion-bonding process.

When the polymer fiber or the polymer nanofiber each is formed from aplurality of polymer materials in the present invention, the Tm and theTg of each composite material is a lower Tm value of Tm values of thefibers and a lower Tg value of Tg values of the fibers.

It is also suitable for the polymer fibers to at least contain materialshaving high sharp melt properties from the viewpoint of ease of handlingin the fusion-bonding process. As the materials having high sharp meltproperties, known substances can be used singly or in combination of twoor more kinds thereof as appropriate and, for example, a hot melt agentand a low melt agent can also be suitably used. Specifically, as the hotmelt agent, PES310S30, PES360S30, and PES375S40 (all manufactured byTOAGOSEI CO., LTD.), 3738, 3747, 3762, and 3764 (all manufactured by3M), and the like can be used singly or in combination. As the low meltagent, 3762LM, 3776LM, 3792LM, and 3798LM (all manufactured by 3M) andthe like can be used singly or in combination.

Polymer Nanofiber as Coating Fiber

The materials of the polymer nanofibers as the coating fibers in thepresent invention are not particularly limited and may be fibers atleast containing polymer materials and organic polymers are suitable. Asthe organic polymers, known polymer materials can be used asappropriate. Polymer materials containing fine particles or a knownfiller and the like may be acceptable and the materials can beconstituted combining the same as appropriate.

The polymer nanofibers according to an embodiment of the presentinvention have at least one or more kinds of polymers and have a lengthlonger than the thickness of the polymer nanofibers.

The polymer nanofibers are suitably thinner than the polymer fiber asthe core fiber. With respect to the thickness of the polymer nanofibers,the average diameter is suitably 1 nm or more and less than 1 μm fromthe viewpoint of coating the core fiber to increase the specific surfacearea. The average diameter is more suitably 30 nm or more and less than1 μm.

In coating the peripheral surface of the core fiber with the polymernanofibers, when the polymer nanofibers are thicker than the core fiber,handling of the polymer nanofibers is difficult from the viewpoint ofmanufacturing, which makes it difficult to coat the peripheral surfaceof the core fiber with the polymer nanofibers again in some cases.Therefore, the polymer nanofibers are suitably thinner than the polymerfiber as the core fiber and suitably have an average diameter of lessthan 1 μm and, more specifically, those which are 10% or more thinnerthan the polymer fibers are suitable.

In general, the polymer nanofibers having an average diameter of lessthan 1 nm need to be manufactured by special techniques, such as aself-assembly method and a phase separation method, and the cost formass production thereof tends to be high. Therefore, the thickness ofthe polymer nanofibers is suitably 1 nm or more.

The cross-sectional shape of the polymer nanofibers is not particularlylimited and may be a circle, an oval, a quadrangle, a polygon, asemicircle, and the like or may not be an exact shape and may havedifferent shapes in an arbitrary cross section.

The thickness of the polymer nanofibers refers to the diameter of thecircle of the cross section in one in which the cross section of thepolymer nanofibers has a circular shape but, in other cases, thethickness refers to the length of the longest straight line passingthrough the center of gravity in the polymer nanofiber cross section.The length of the polymer nanofibers is 10 or more times the thicknessthereof.

The polymer material to be used in the polymer nanofibers according tothe present invention is not particularly limited but is limited only interms of the relationship with the polymer material to be used as thepolymer fibers described above and the polymer material of the polymerfibers can be used as appropriate.

It is suitable for the polymer nanofibers coating the polymer fibers toat least contain the polymer material constituting the polymer fibers.

From the viewpoint of maintaining the shape of the polymer nanofibers inthe fusion-bonding process, it is also suitable to use polymermaterials, such as polyimide, polyamide, polyamide imide (PAI), andpolybenzimidazole (PBI) which are hard to be melt, singly withoutcombining with thermoplastic resin and the like, for example. Morespecifically, polyimide, polyamide, polyamide imide (PAI),polybenzimidazole (PBI), and the like are referred to as an engineeringplastic and have a Tg higher than the Tm of general thermoplasticpolymer materials. Therefore, the use of these substances is suitablebecause the shape of the polymer nanofibers tends to easily maintain inthe fusion-bonding process.

Coating Amount of Polymer Nanofibers Coating Polymer Fiber

The amount of the polymer nanofibers coating the polymer fiber as thecore fiber may be adjusted as appropriate according to desiredperformance and, for example, is 1 part by weight or more and 95 partsby weight or less and suitably 5 parts by weight or more and 90 parts byweight or less based on 100 parts by weight of the polymer fibers. Theamount of the polymer nanofibers of less than 1 part by weight is notsuitable because an increase in specific surface area is insufficient insome cases. When the amount of the polymer nanofibers exceeds 95 partsby weight, the polymer fibers cannot be fusion-bonded to each other inat least one portion in some cases, and therefore the amount is notsuitable.

Method for Manufacturing Fiber Material According to the PresentInvention

Next, a method for manufacturing the fiber material according to thepresent invention is described.

The method for manufacturing the fiber material includes a spinningprocess including coating a polymer fiber as a core fiber containing apolymer material with polymer nanofibers containing a polymer materialto obtain a nanofiber coated polymer fiber, and a fusion-bonding processincluding fusion-bonding the polymer fiber and the polymer nanofibersand also fusion-bonding two or more of the polymer fibers in at leastone portion.

Specifically, the method for manufacturing the fiber material accordingto the present invention includes a spinning process including coating apolymer fiber as a core fiber containing a polymer material with polymernanofibers containing a polymer material to obtain a nanofiber coatedpolymer fiber and a fusion-bonding process including fusion-bonding thepolymer fiber and the polymer nanofibers coating the polymer fibers andalso fusion-bonding two or more of polymer fibers forming the fibermaterial in at least one portion.

The method for manufacturing the fiber material according to the presentinvention is not particularly limited and, for example, a methodincluding performing bicomponent spinning of the polymer fibers and thepolymer nanofibers by an electrospinning method (an electric fieldspinning method and an electrostatic spinning method), and then heatingthe resultant fibers in an oven for fusion-bonding the fibers to eachother and the polymer nanofibers and the polymer fibers or, in place ofthe bicomponent spinning using the electrospinning method singly, theelectrospinning method, a melt blow method, and the like may be used incombination.

The electrospinning method is a method for manufacturing polymer fibersin which a high voltage is applied between a polymer solution in asyringe and a collector electrode to charge the solution forced out ofthe syringe, and then the solution is dispersed to be formed into thinlines to form polymer nanofibers, and the polymer nanofibers adhere tothe collector.

Among the manufacturing methods described above, it is suitable toperform the bicomponent spinning by the electrospinning method becausespinning of various polymers into a fiber shape can be performed,control of the fiber shape is relatively easy, fibers having an averagediameter ranging from several tens μm to a nanosize can be easilyobtained, and also the manufacturing process is simple.

The spinning process using the bicomponent spinning of the polymerfibers and the polymer nanofibers by the electrospinning method which isone embodiment of the present invention is described with reference toFIG. 2.

FIG. 2 is a schematic view illustrating one embodiment of the method formanufacturing the fiber material of the present invention. Inparticular, FIG. 2 is a schematic view illustrating the bicomponentspinning using the electrospinning method which is an example of atechnique for manufacturing the nanofiber coated polymer fiber of thepresent invention.

As illustrated in FIG. 2, the bicomponent spinning is performed using ahead 17 in which a plurality of polymer solution storage tanks 12, 13,and 14 are disposed through a connection portion 11 connected to ahigh-pressure power supply 16 and a grounded collector 15. The referencenumeral 10 denotes a device for manufacturing the fiber material.

The polymer solution is forced out of the tanks 12, 13, and 14 tospinning orifices 22, 23, and 24 at a fixed speed. A voltage of 1 to 50kV is applied to each of the spinning orifices 22, 23, and 24. Whenelectric attraction exceeds the surface tension of the polymer solution,polymer solution jets 18, 19, and 20 are ejected to the collector 15. Inthis process, the solvent in the jets is gradually volatilized, andthen, when reaching the collector, corresponding fibers are obtained.Herein, the polymer solutions whose conditions are set to the conditionsunder which the polymer solutions are formed into polymer nanofibers areintroduced into the tanks 12 and 13, and, on the other hand, a polymersolution whose conditions are set to the conditions under which thepolymer solution is formed into polymer fibers is introduced into thetank 14, and then bicomponent spinning is performed.

Herein, a melted polymer heated to be equal to or higher than themelting point may be utilized instead of the polymer solution.

The polymer nanofibers and the polymer fibers can be confirmed by directobservation using a scanning electron microscope (SEM) or lasermicroscope measurement.

The average fiber diameter (average diameter) of the polymer nanofibersor the polymer fibers can be determined by measuring the concernedcomposite fiber film under a scanning electron microscope (SEM),capturing an image thereof into image analysis software “Image J”, andthen measuring the width at arbitrary 50 points of the polymernanofibers or the polymer fibers.

Fusion-Bonding Process of Polymer Nanofibers and Polymer Fibers, andPolymer Fibers

The description “fusion-bonding the polymer fibers and the polymernanofibers coating the polymer fibers” or “fusion-bonding the pluralityof polymer fibers forming the fiber material to each other in at leastone portion” in the present invention refers to a state where at leastthe polymer fibers are softened to adhere to the polymer nanofiberscoating the polymer fibers or adjacent polymer fibers, so that theadhesion boundary portion has a sheet shape or the adhesion boundary islost.

The fusion-bonding process is not particularly limited and may bethermal fusion-bonding, ultrasonic fusion-bonding, and frictionfusion-bonding and also fusion-bonding (hotpress) by thermocompressionbonding and is suitably thermal fusion-bonding in terms of ease ofhandling. The thermal fusion-bonding method is not particularly limitedand, for example, a hot pressing method, a method for performingfusion-bonding by heating with an industrial drier, oven, or the like, amethod for performing heating with a heater once, and then furtherheating in an oven for fusion-bonding, and the like can be suitablyused. Among the above, a technique of performing fusion-bonding using anoven can be particularly suitably used because the temperature of theentire material can be easily made uniform without unevenness.

The temperature of the fusion-bonding is not particularly limitedinsofar as the temperature is equal to or less than the decompositiontemperature of the polymer material and may be selected as appropriateaccording to the polymer material to be used, a desired fiber material,and the like. The temperature of heating is suitably 30 to 250° C. andmore suitably near the melting point of at least the polymer fibers orboth the polymer fibers and the polymer nanofibers. As described above,when the temperature of the fusion-bonding is equal to or higher thanthe melting point (Tm) of the polymer material forming the polymerfibers and is equal to or less than the glass transition point (Tg) ofthe polymer material forming the polymer nanofibers, the shape of thepolymer nanofibers is easily maintained and thus the temperature of thefusion-bonding is very suitable. The heating time is suitably 0.1 minuteto 60 minutes.

A method for confirming “the fusion-bonding of the polymer fibers andthe polymer nanofibers coating the polymer fibers” and “thefusion-bonding of the polymer fibers (the polymer fibers arefusion-bonded to each other in at least one portion)” can be performedin direct observation using a laser microscope or an electron microscope(SEM) before and after the fusion-bonding process.

More specifically, the polymer fibers are fusion-bonded to each other inat least one portion, i.e., the fusion-bonding of the polymer fibers inat least one portion, in the present invention refers to a state wherethe polymer fibers are softened to adhere to adjacent polymer fibers, sothat the adhesion boundary portion has a sheet shape or the adhesionboundary is lost. Therefore, when the state is confirmed in which thepolymer fibers or the polymer nanofibers are softened, so that theadjacent polymer fibers or the polymer fibers and the polymer nanofiberscoating the same adhere to each other, so that the adhesion boundaryportion has a sheet shape or the adhesion boundary is lost, it can beconfirmed that the fibers are fusion-bonded.

FIG. 3 shows an optical microscope photograph of a nanofiber coatedpolymer fiber material of Example 5 as an example showing thefusion-bonding of the polymer nanofibers and the polymer fibers and thefusion-bonding of the polymer fibers according to the present invention.In FIG. 3, it can be confirmed that polymer nanofibers having an averagediameter of about 700 nm formed from polyamide imide (PAI) arefusion-bonded to the surface of the polymer fibers having an averagediameter of about 35 μm formed from a polyester material (PES) to coatthe polymer fibers and adjacent polymer fibers have a structure that thepolymer fibers are fusion-bonded to each other in at least one portion.

Evaluation of Fusion-Bonding Degree of Polymer Fiber and PolymerNanofibers Coating Polymer Fiber

The evaluation of the fusion-bonding degree of the polymer nanofiberscoating the polymer fibers in the present invention was performed usinga simple tape peeling test including bonding a pressure sensitiveadhesive tape (NICHIBAN CO., LTD.: CT-18, 0.401 N/mm) to both surfacesof a manufactured fiber material, and then vertically tearing the sameoff using an instron tester (Shimadzu: EZ-test).

More specifically, arbitrary 10 points (observation points) on the fibermaterial surface were marked beforehand, the peeling degree of thepolymer nanofibers coating the polymer fibers before and after thesimple tape peeling test at the observation points was observed under alaser microscope, and then the peeling degree was evaluated in thefollowing three grades of A to C:

A: At all the observation points, peeling of the polymer nanofiberscoating the polymer fibers is not observed;B: At 1 to 4 observation points, peeling of the polymer nanofiberscoating the polymer fibers is observed;C: At 5 or more observation points, peeling of the polymer nanofiberscoating the polymer fibers is observed.

The fusion-bonding degree is higher in order of A>>B>C, and, as aresult, the peeling resistance between the polymer nanofibers and thepolymer fiber is high. Therefore, polymer nanofiber coated polymer fibermaterials free from easy peeling and detachment of the polymernanofibers due to an external factor, such as rubbing, leading to areduction in specific surface area of the fiber material are obtained inorder of A>>B>C.

Evaluation of Fusion-Bonding Degree of Polymer Fibers

An increase in mechanical strength of the fiber material due to thefusion-bonding of the polymer fibers in the nanofiber coated polymerfiber material can be easily confirmed by measuring the Young's modulusof the corresponding fiber materials before and after the fusion-bondingprocess.

The evaluation of the fusion-bonding degree of the polymer fibers in thepresent invention is performed by measuring changes in mechanicalstrength of the fiber materials before and after the fusion-bondingprocess using tensile property measurement employing an autograph(AG-Xplus) manufactured by Shimadzu.

Specifically, it is confirmed in samples before and after thefusion-bonding process that the polymer fibers are fusion-bonded to eachother in at least one portion by direct observation using a lasermicroscope or an electron microscope (SEM). Thereafter, the Young'smodulus of the fiber material in a state where the polymer nanofibersmerely physically coat the polymer fibers and the Young's modulus of thesamples after the fusion-bonding process are measured, and then theincrease percentage of the Young's modulus is calculated.

When the increase percentage of the Young's modulus is higher, thefusion-bonding degree in which the polymer fibers are fusion-bonded toeach other in at least one portion is higher. As a result, themechanical strength of the fiber material improves, and therefore thefiber material can be used over a long period of time. Evaluation ofshape changes in polymer nanofibers before and after fusion-bondingprocess

The shape changes in the polymer nanofibers before and after thefusion-bonding process in the present invention can be performed using ascanning electron microscope (SEM).

More specifically, the shape change rate in the polymer nanofibers iscalculated by observing the fiber material before and after thefusion-bonding process observed using an SEM, capturing images thereofinto image analysis software “Image J”, individually measuring the fiberwidths of the polymer nanofibers at arbitrary 50 points from a direction(upper surface) perpendicular to the thickness direction of the fibermaterial before and after the fusion-bonding process, and then comparingthe fiber widths before and after the fusion-bonding. Then, the degreeof the shape change rate was evaluated in the following three stages ofI to III based on the results: I: Changes in the polymer nanofibers arenot observed at all the observation points;

II: Changes in the polymer nanofibers are slightly observed (within +5to +10%);III: Changes in polymer nanofibers are very noticeably observed (largerthan +10%).

EXAMPLES

Hereinafter, Examples of the present invention are described.

Examples of the present invention are described in detail below but aremerely examples and do not limit the present invention. The technique ofthe present invention also includes various modifications andalternations of specific examples described below.

Example 1

This example relates a fiber material in which polymer fibers are formedfrom polycapro lactone (PCL) and polymer nanofibers are formed frompolyethylene oxide (PEO). The fiber material is manufactured as follows.

First, 1 mL of a PCL diluted solution A adjusted to 8 wt % usingpolycapro lactone (PCL, Weight average molecular weight of 80000,manufactured by Aldrich) and a solution obtained by mixing THF(tetrahydrofuran) and DMF (dimethyl formamide) at 6:4 (Volume ratio) isused. 2 mL of a PEO diluted solution B in which polyethylene oxide(manufactured by Aldrich) is adjusted to 6 wt % using pure water isused.

Next, these diluted solutions are simultaneously ejected for performingbicomponent spinning by an electrospinning method, whereby nanofibercoated polymer fibers in which the PCL polymer fibers as core fibers arephysically coated with the PEO polymer nanofibers is obtained.

More specifically, a head (Clip spinneret, FIG. 2) capable of performingspinning of a plurality of solutions is attached to an electrospinningdevice (manufactured by MEC Co., Ltd.).

Next, a tank which is filled with the PCL diluted solution A is attachedto the center of the head and a tank filled with the PEO dilutedsolution B is attached to both ends. Then, the device is moved from sideto side at 50 mm/s while applying a voltage of 19 kV to spinningorifices to thereby eject each diluted solution to a collector. Then, byejecting the solutions for 15 minutes, nanofiber coated polymer fibersin which the PCL polymer fibers are coated with the PEO polymernanofibers can be obtained.

Then, the obtained nanofiber coated polymer fibers are held betweenglass plates, put into an oven, and then heat-treated at 65° C. for 0.5minute, whereby a fiber material in which the polymer fibers and thepolymer nanofibers are fusion-bonded and the polymer fibers adjacent toeach other are fusion-bonded to each other in at least one portion isobtained.

The thickness of the polymer fibers in the obtained fiber material thusobtained is 18 μm in average diameter. The thickness of the polymernanofibers is 650 nm in average diameter.

Example 2

This example is a modification of Example 1 and is manufactured in thesame manner as in Example 1, except changing the materials of thepolymer fibers and the polymer nanofibers and changing the followingconditions.

As the material of the polymer fibers, 1 mL of a PS/PET diluted solutionC in which a mixture (Volume ratio of 9:1) of polystyrene (PS, Weightaverage molecular weight of 280000, manufactured by Aldrich) andpolyethylene terephthalate (PET, Weight average molecular weight of10000, manufactured by Aldrich) is adjusted to a 30 wt % solution usingDMF is used. As the material of the polymer nanofibers, 2 mL of a PETdiluted solution D in which PET is adjusted to 13 wt % using a solutionin which dichloromethane (DCM) and trifluoroacetic acid (TA) are mixedwith 1:1 (volume ratio) is used.

The application of a voltage to the spinning orifices is performed at 22kV. The fusion-bonding process is performed by heat-treatment in an ovenat 260° C. for 0.5 minute, whereby a fiber material in which the polymerfibers and the polymer nanofibers are fusion-bonded and the polymerfibers adjacent to each other are fusion-bonded to each other in atleast one portion is obtained.

The thickness of the polymer fibers in the obtained fiber material thusobtained is 1.5 μm in average diameter. The thickness of the polymernanofibers is 300 nm in average diameter.

Example 3

This example is a modification of Example 1 and is manufactured in thesame manner as in Example 1, except changing the materials of thepolymer fibers and the polymer nanofibers and changing the followingconditions.

As the material of the polymer fibers, 1 mL of a PVDF-HFP dilutedsolution E in which polyvinylidene fluoride-hexafluoro propylene(PVDF-HFP) (KYNAR 2750, manufactured by KYNAR) is adjusted to a 10 wt %solution using dimethyl acetamide (DMAc) is used.

As the material of the polymer nanofibers, a mixture of polyvinylidenefluoride (PVDF, KYNAR 460, manufactured by KYNAR) and montmorillonite(MN, nanoclay, manufactured by Aldrich) is used. This mixture ismanufactured by the following procedure.

10 mg of montmorillonite (MN) as a filler and 1 mL of organic solvent(DMAc) are put in a container. Zirconia balls having a particle diameterof 2 mm are added to ⅓ of the capacity of the container, and then themixture is dispersed under the conditions of 200 rpm/30 minutes using aball mill machine (manufactured by Fritsch, Planetary pulverizer).Subsequently, a solution in which 200 mg of PVDF as a base material isdissolved in 2 mL of DMAc is added, and then further dispersed under theconditions of 500 rpm/60 minutes. Thus, 2 mL of a PVDF-MN diluted mixedsolution E which is a material of polymer nanofibers is prepared.

The application of a voltage to the spinning orifices is performed at 18kV. The fusion-bonding process is performed by heat-treatment in an ovenat 155° C. for 0.5 minute, whereby a fiber material in which the polymerfibers and the polymer nanofibers are fusion-bonded and the polymerfibers adjacent to each other are fusion-bonded to each other in atleast one portion is obtained.

The thickness of the polymer fibers in the obtained fiber material thusobtained is 20 μm in average diameter. The thickness of the polymernanofibers is 220 nm in average diameter.

Example 4

This example is a modification of Example 1 and is manufactured in thesame manner as in Example 1, except changing the materials of thepolymer fibers and the polymer nanofibers and changing the followingconditions.

The material of the polymer fibers is adjusted as follows.

In a container, 0.1 wt % of silica particles (SiO₂, nanopowder, Averageparticle diameter of 10 to 20 nm, manufactured by Aldrich) is added to10 parts by weight of a thermoplastic polyester hot melt material (PES,Aronmelt PES375S40, manufactured by TOAGOSEI CO., LTD., Solid content of40 wt %, Solvent (toluene:MEK=8:2)), and then mixed. Next, Zirconiaballs having a particle diameter of 2 mm are added to ⅓ of the capacityof the container, and then the mixture is dispersed under the conditionsof 500 rpm/30 minutes using a ball mill machine (manufactured byFritsch, Planetary pulverizer) to prepare 1 mL of a PES-SiO₂ dilutedmixed solution E.

As the material of the polymer nanofibers, 2 mL of a PBI dilutedsolution F in which polybenzimidazole (PBI, manufactured by Aldrich) isadjusted to 20 wt % in a DMAc solution in which 4 wt % of lithiumchloride (LiCl) is dissolved is used.

The application of a voltage to the spinning orifices is performed at 23kV. The fusion-bonding process is performed by heat-treatment in an ovenat 130° C. for 1 minute, whereby a fiber material in which the polymerfibers and the polymer nanofibers are fusion-bonded and the polymerfibers adjacent to each other are fusion-bonded to each other in atleast one portion is obtained.

The thickness of the polymer fibers in the obtained fiber material thusobtained is 40 μm in average diameter. The thickness of the polymernanofibers is 150 nm in average diameter.

Example 5

This example is a modification of Example 1 and is manufactured in thesame manner as in Example 1, except changing the materials of thepolymer fibers and the polymer nanofibers and changing the followingconditions.

As the material of the polymer fibers, 1 mL of a thermoplastic polyesterhot melt material (PES, Aronmelt PES375S40, manufactured by TOAGOSEICO., LTD., Solid content of 40 wt %, Solvent (toluene:MEK=8:2) isprepared.

As the material of the polymer nanofibers, one in which polyamide imide(PAI, pyromax HR-13NX) is adjusted to a solid content concentration of25 wt % using DMF is used.

The application of a voltage to the spinning orifices is performed at 19kV. The fusion-bonding process is performed by heat-treatment in an ovenat 130° C. for 1 minute, whereby a fiber material in which the polymerfibers and the polymer nanofibers are fusion-bonded and the polymerfibers adjacent to each other are fusion-bonded to each other in atleast one portion is obtained.

The thickness of the polymer fibers in the obtained fiber material thusobtained is 35 μm in average diameter. The thickness of the polymernanofibers is 700 nm in average diameter. FIG. 3 shows an opticalmicroscope photograph of the fiber material of Example 5 of the presentinvention. The length between two points indicated by “1” in FIG. 3 is33 μm.

Comparative Example 1

This comparative example is a modification of Example 1 and ismanufactured in the same manner as in Example 1, except performing thefusion-bonding process in Example 1.

The thickness of the polymer fibers in the obtained fiber material thusobtained is 9 μm in average diameter. The thickness of the polymernanofibers is 500 nm in average diameter.

Table 1 shows the melting point (Tm) and the glass transition point (Tg)of the polymer materials used in Examples and Comparative Example.

TABLE 1 Melting Glass transition point Polymer point (Tm) (Tg) material(° C.) (° C.) PCL 60 −60 PEO 65 −73 PS 230 100 PET 260 80 PVDF-HFP 130to 138 −42 to −40 PVDF 155 to 160 −40 to −38 PES 130 23 PBI None 473 PAINone 290

Evaluation of Performance of Fiber Material

It was observed by SEM that the fiber materials in Examples 1 to 5 havea structure in which the polymer fibers and the polymer nanofiberscoating the polymer fibers are fusion-bonded and a plurality of polymerfibers forming the fiber materials are fusion-bonded to each other.

The peeling resistance between the nanofibers and the core fiber wasexamined by “Evaluation of fusion-bonding degree of polymer fiber andpolymer nanofibers coating polymer fiber” performed by the tape peelingtest described above.

The mechanical strength of the fiber materials was examined by“Evaluation of fusion-bonding degree of polymer nanofiber coated polymerfibers” by a tensile strength test.

The fiber materials with a large specific surface area are obtained bythe coating with the polymer nanofibers. It was examined that thepolymer nanofibers coating the polymer fibers are maintained before andafter the fusion-bonding (maintenance of unevenness of nanofiber coatedpolymer fibers) by “Evaluation of shape changes in polymer nanofibersbefore and after fusion-bonding process”.

Table 2 shows the used materials, the peeling test results, the strengthincrease rate, and the degree of shape changes in the polymer nanofibersbefore and after the fusion-bonding process in Examples and ComparativeExample.

TABLE 2 Degree of shape Polymer Strength changes in fiber increase ratepolymer material (After fusion- nanofibers Polymer Peelingbonding/Before (After fusion- nanofiber test fusion- bonding/Beforematerial results bonding) % fusion-bonding) Ex. 1 PCL B 300 II (30%) PEOEx. 2 PS + PET A 400 II (13%) PET Ex. 3 PVDF-HFP A 600 II (10%) PVDF/MNEx. 4 PES/SiO₂ B 700 I (0%) PBI Ex. 5 PES B 650 I (0%) PAI Comp. PCL C —— Ex. 1 PEO

Evaluation of Fusion-Bonding Degree of Polymer Fiber and PolymerNanofibers Coating Polymer Fiber

From the results of Examples 1 to 5 and Comparative Example 1, inExamples 1 to 5, by fusion-bonding the polymer fibers and the polymernanofibers coating the polymer fibers, the polymer nanofibers do notpeel (A) or very slightly peel (B) in the tape peeling test, so that aremarkable performance improvement of the peeling resistance between thenanofibers and the core fiber can be confirmed.

In particular, from Example 1 and Comparative Example 1, byfusion-bonding the polymer fibers and the polymer nanofibers coating thepolymer fibers, a remarkable performance improvement can be confirmed inthe tape peeling test (C→B) also in the same polymer fibers/polymernanofiber material system.

In addition thereto, from Example 2 and Example 3, when the same polymermaterial (the structure constituting the polymer material is the same)is contained in the material of the polymer fibers and the material ofthe polymer nanofibers, the polymer nanofibers do not peel (A) in thetape peeling test by fusion-bonding the polymer fibers and the polymernanofibers coating the polymer fibers, so that a remarkable performanceimprovement of the peeling resistance between the polymer nanofibers andthe core fiber remarkably can be confirmed.

Therefore, it is found that the fiber material in the present inventionhas high peeling resistance between the polymer nanofibers and the corefiber. More specifically, ease peeling and detachment of the polymernanofibers due to an external factor, such as rubbing, leading to areduction in the specific surface area of the fiber material does notoccur.

Evaluation of Fusion-Bonding Degree of Polymer Fibers

The plurality of polymer fibers forming the fiber materials of Examples1 to 5 are structured to be fusion-bonded to each other. Therefore, itcan be confirmed that the tensile strength improves several times beforeand after the fusion-bonding and the mechanical strength of the fibermaterial remarkably improves.

When a fiber material was constituted using materials, such as aninorganic filler (MN, SiO₂) or an engineering plastic (PBI, PAI), thetendency which shows a large improvement of mechanical strength beforeand after the fusion-bonding process was able to be confirmed (Examples3 to 5).

Therefore, it is found that the mechanical strength of the fibermaterial in the present invention is high. More specifically, the corefibers are not easily loosened and the fiber material is advantageousfor long-term use.

Evaluation of Shape Changes in Polymer Nanofibers Before and afterFusion-Bonding Process

It is found from the results of Examples 1 to 3 that when the meltingpoint (Tm₁) of the polymer material forming the polymer fibers is lessthan the melting point (Tm₂) of the polymer material forming polymernanofibers and the temperature difference (Tm₂−Tm₁) is 5° C. or higher(Example 1: PCL (Tm₁: 60° C.), PEO (Tm₂: 65° C.)), the shape of thepolymer nanofibers before and after the fusion-bonding process isfavorably maintained (II).

It can be confirmed from the results of Example 2 and Example 3 thatwhen the temperature difference (Tm₂−Tm₁) is 30° C. or higher (Example2: PS (Tm₁: 230° C.), PET (Tm₂: 260° C.), Example 3: PVDF-HFP (Tm₁: 130°C.), and PVDF (Tm₂: 160° C.)), the change rate is small, i.e., less than10%.

It can be confirmed from the results of Example 4 and Example 5 thatwhen the Tm₁ of the polymer material forming the polymer fibers is equalto or less than the glass transition point (Tg) of the polymer materialforming the polymer nanofibers (Example 4: PES (Tm₁: 130° C.), PBI (Tg:473° C.), Example 5: PES (Tm₁: 130° C.), PAI (Tg: 290° C.)), the shapeof the polymer nanofibers can be maintained in the fusion-bondingprocess.

When the melting temperature (Melting point: Tm₁) of the polymermaterial forming the polymer fibers is less than Tm₂ of the polymermaterials forming the polymer nanofibers and the difference is large,the fusion-bonding of the polymer fibers and the polymer nanofiberscoating the same can be performed by preferentially fusion-bonding thepolymer fiber side.

Therefore, according to the present invention, a fiber material can bemanufactured in which the polymer fibers and the polymer nanofiberscoating the polymer fibers are fusion-bonded while maintaining the shape(unevenness) of the polymer nanofibers, and therefore an increase effectof the specific surface area based on the coating with the polymernanofibers is easily demonstrated.

As shown in each Example above, according to the configuration of thepresent invention, a fiber material coated with nanofibers in which thepeeling strength between the polymer nanofibers and the polymer fiber asthe core fiber and the mechanical strength of the fiber material arehigh and the specific surface area is large can be provided.

INDUSTRIAL AVAILABILITY

The nanofiber coated fiber material of the present invention can be afiber material having a large specific surface area which can be usedover a long period of time even when an external factor, rubbing, isapplied, and therefore can be suitably utilized, for example, as afriction charging material in a static electricity generator and aparticle electric field separator.

As described above with reference to Embodiments and Examples, thepresent invention can provide a nanofiber coated fiber material in whichthe peeling resistance between nanofibers and a core fiber is excellentand the mechanical strength of the fiber material is high and a methodfor manufacturing the same.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. A process for producing a fiber material, thefiber material, comprising: an assembly of a plurality of nanofibercoated polymer fibers in which a polymer fiber serving as a core fiberis coated with polymer nanofibers, the polymer fiber and the polymernanofibers being bonded such that an adhesion boundary portion betweenthe polymer fiber and the polymer nanofibers has a sheet shape or islost, two or more of the polymer fibers being bonded to each other in atleast one portion such that an adhesion boundary portion between the twoor more of the polymer fibers has a sheet shape or is lost, and anaverage diameter of the polymer fiber being 10 μm to 50 μm, and anaverage diameter of the polymer nanofibers being 1 nm or more and lessthan 1 μm, the process comprising the steps of: (i) extruding polymersolution or melted polymer from spinning orifices simultaneously towardsa collector, (ii) forming the electro-spun polymer fiber andelectro-spun polymer nanofibers, (iii) coating the electro-spun polymerfiber with the electro-spun polymer nanofibers to form an electro-spunpolymer fiber coated with the electro-spun polymer nanofibers, and (iv)heating the electro-spun polymer fiber coated with the electro-spunpolymer nanofibers, and bonding the electro-spun polymer fiber and theelectro-spun polymer nanofibers each other so that an adhesion boundaryportion between the electro-spun polymer fiber and the electro-spunpolymer nanofibers has a sheet shape or is lost.
 2. The processaccording to claim 1, wherein an amount of the electro-spun polymernanofibers of the electro-spun polymer fiber coated with theelectro-spun polymer nanofibers, is 5 parts by weight or more and 90parts by weight or less.
 3. The process according to claim 1, whereintemperature of the heating in the step (iv) is equal to or higher thanthe melting point of the polymer material forming the electro-spunpolymer fiber, and is equal to or less than the glass transition pointof the polymer material forming the electro-spun polymer nanofibers.